In most of the conventional arts, in order to extract the placer gold etc. from the sand layer in rivers (including lakes and marshes as well as rivers) containing the placer gold etc., the placer gold etc. are separated by washing the sand taken up by human labor, and the placer gold etc. are separated by elutriating the sands and like collected by excavating the sand layer containing the placer gold etc.
The excavation described above is performed by using machinery, and no excavating means is available in cases where the layer containing the placer gold etc. is located deep in the earth. In such cases, it would be possible to employ the existing technologies, for example, if a large hole is made by digging the earth deeply, and only the sand layer containing the placer gold etc. is excavated, separated and transported, but this method is not cost-justified. Thus, any layer containing the placer gold etc. that is located deep in the earth is left untouched.
Since the placer gold etc. are separated by elutriating the sands collected by excavating the sand layer containing the placer gold etc. as described above, extensive facilities are required, but it is difficult to improve the efficiency. No epoch-making method for separating the placer gold etc. has yet been proposed.
The conventional elutriation that is performed by the human labor not only involves heavy labor, but it is also not efficient, and there is a problem in that it is not cost-justified for those sand layers which contain a smaller proportion of the placer gold etc. When the sands obtained by the mechanical digging are elutriated, an enormous amount of sands must be elutriated, and it would not be cost-justified except for those sand layers which contain a relatively large proportion of the placer gold etc. It would only be cost-justified for those sand layers, in particular, which contain a very large amount of the placer gold etc.
The layers that are at present known as containing the placer gold etc. are presumed to be deposited 1000 m or more down from the ground surface level, and the excavation is currently limited to the depth of 5 m to 10 m from the gound surface leve. For the regions where the undergound water level is located 5 m to 10 m deep, in particular, it is thought that the cost justification cannot be realized when the sand layer is excavated more deeply than the underground water level. Such regions are now wasted. In other words, the excavating method and the transporting method that are now available would not be cost-justified for those layers containing the placer gold etc. which are located 1000 m or more down from the ground surface level.
The conventional separation by elutriation that makes use of the difference in the specific gravity of the placer gold etc. and other mineral substances (i.e., the so-called selection by specific gravity) consists of letting the placer gold etc. drop together with water over the wide region, and separating the placer gold etc. by elutriation. This separation process is not efficient, and has problems in that it requires not only a large area for separation, but also lots of labor since human labor is involved in collecting the placer gold etc. separated by elutriation when this process occurs on the mass production basis. This leads to a higher production cost. The conventional separation by elutriation, which is based on the selection by specific gravity, employs the natural dropping (by the head of water) system. The forced delivery that occurs in large quantities by using a pump and the like has not been proposed yet.